Lennox SLP98UHV Error Code E 311: Heat Rate Reduced - Cutback Mode
What Does Code E 311 Mean?
Error code E311 on the Lennox SLP98UHV indicates that the furnace blower is in cutback mode due to restricted airflow. The control board has automatically reduced the heat rate (firing rate) to match the available airflow. Your furnace is still running, but at lower capacity than normal.
The SLP98UHV is a variable-capacity furnace that can adjust its firing rate across a wide range. When the ECM blower motor detects that airflow is below the optimal level — but not yet critically low — the control board reduces the gas input to prevent overheating. This is actually a smart safety feature that keeps the furnace running at reduced output rather than shutting it down entirely.
The most common cause is a dirty air filter. As the filter loads up with dust, the blower cannot push enough air through the system, and the control board compensates by lowering the heat output. A less common cause is undersized or restricted ductwork that limits the maximum airflow the system can achieve. If left uncorrected, E311 can eventually escalate to E291 (critically low airflow) as the filter gets dirtier.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Dirty air filter restricting airflow | Most common | ✓ DIY fix → |
| Undersized or restricted ductwork | Common | ✗ Call a pro → |
How to Fix It: Replace the Air Filter
What You'll Need
- Replacement air filter (matching size) 🛒 Find at FiltersFast · 🛒 Find at Amazon
Steps
- Turn off electrical power at the breaker and shut off the gas supply valve Locate the furnace circuit breaker and flip it to OFF. Turn the gas shutoff valve to the OFF position (handle perpendicular to the pipe). If you smell gas, leave immediately and call your gas company.
- Replace the air filter Locate the air filter in the blower compartment or return air duct. Remove the dirty filter and replace it with a new one of the correct size. Ensure the airflow direction arrow points toward the blower. If you are using a high-MERV filter (MERV 13 or above), consider switching to a lower-MERV filter as recommended by Lennox, since higher filtration creates more resistance.
- Open all supply registers and return grilles Check every room to ensure all vents are open and unobstructed. Even a few closed registers can cause enough airflow reduction to trigger cutback mode on a variable-capacity furnace.
- Restore power and gas, then test Turn the gas supply valve to ON (handle parallel to the pipe) and flip the circuit breaker to ON. Set your thermostat to call for heat. The furnace should ramp up to its normal firing rate now that airflow is restored.
When to Call a Professional
Contact a licensed HVAC technician if:
- E311 persists after replacing the filter and opening all vents
- The furnace also shows E291 (critically low airflow)
- You suspect the ductwork is undersized for the furnace capacity
- The blower wheel appears dirty or damaged
- Heating output remains low even after addressing airflow restrictions
✓ Verified against manufacturer service manual — March 2026